Control arrangement for a resonant mode power converter

ABSTRACT

A resonant mode power converter is controlled with a control unit including a current limiting circuit coupled to receive a first current representative of a power converter output and a second current generated in response to a reference voltage. The current limiting circuit is coupled to limit the first current in response to the second current. An oscillator is coupled to receive the first current to generate a control signal having a control frequency in response to the first current. The power converter output is controlled in response to the control frequency of the control signal.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 12/939,058, filed Nov. 3, 2010, now pending, whichis a continuation of U.S. Non-Provisional patent application Ser. No.12/016,933, filed Jan. 18, 2008, now issued as U.S. Pat. No. 7,848,117B2, which claims the benefit and priority of U.S. Provisional PatentApplication No. 60/881,480, filed Jan. 22, 2007, entitled “CascadedPower Converters And Control Arrangement Therefor,” which is nowexpired. The U.S. Provisional Patent Application No. 60/881,480, U.S.Non-Provisional patent application Ser. No. 12/939,058 and U.S. Pat. No.7,848,117 B2 are hereby incorporated by reference.

Reference is directed to the following U.S. Non-Provisional patentapplication Ser. Nos. 12/016,950 and 12/016,945 filed simultaneouslywith the U.S. Non-Provisional patent application Ser. No. 12/016,933,referenced above, claiming separate inventions, the entire contents anddisclosures of each of which is hereby incorporated herein by reference:

“Control Arrangement For A PFC Power Converter,” R. Colbeck et al.,(U.S. Non-Provisional patent application Ser. No. 12/016,950, filed Jan.18, 2008, now issued as U.S. Pat. No. 7,911,812);

“Cascaded PFC And Resonant Mode Power Converters,” R. On et al., (U.S.Non-Provisional patent application Ser. No. 12/016,945, filed Jan. 18,2008, now issued as U.S. Pat. No. 7,885,085).

BACKGROUND INFORMATION

1. Field of the Disclosure

This invention relates to a control arrangement for a resonant modepower converter.

2. Background

It is known to provide a cascade of a boost converter for PFC followedby a PWM (pulse width modulation) buck converter for producing a lowervoltage than the typically high output voltage of the PFC converter, andto operate these in a synchronized manner using a single clockreference. Such cascaded converters are described for example in HwangU.S. Pat. No. 5,565,761, issued Oct. 15, 1996 and entitled “SynchronousSwitching Cascade Connected Off-Line PFC-PWM Combination Power ConverterController”, and Hwang et al. U.S. Pat. No. 5,798,635, issued Aug. 25,1998 and entitled “One Pin Error Amplifier And Switched Soft-Start ForAn Eight Pin PFC-PWM Combination Integrated Circuit ConverterController”.

Another arrangement comprising cascaded PFC and PWM power converters isknown from Fairchild Semiconductor Application Note 42047 entitled“Power Factor Correction (PFC) Basics”, Rev. 0.9.0, Aug. 19, 2004.Various PFC arrangements and their control are known for example fromChapter 1, entitled “Overview of Power Factor Correction Approaches”, of“Power Factor Correction (PFC) Handbook”, ON Semiconductor documentHBD853/D, Rev. 2, August 2004, and from “The Dynamics of a PWM BoostConverter with Resistive Input” by S. Ben-Yaakov et al., IEEETransactions on Industrial Electronics, Vol. 46, No. 3, June 1999, pp.613-619, describing an indirect PFC converter control scheme.

It is desirable for the converter switching frequency to be relativelyhigh, in order to reduce the sizes of reactive components. However,switching losses increase with increasing switching frequency, resultingin practical upper limits to the switching frequencies that can be used.

It is also known to reduce the PWM power converter switching losses byusing a resonant mode power converter, taking advantage of zero voltageswitching (ZVS) and/or zero current switching (ZCS). Examples ofresonant mode converters include series resonant, parallel resonant,series parallel resonant or LCC, and LLC converters examples of whichusing a half bridge converter topology are described in Chapter 4,entitled “LLC Resonant Converter”, of “Topology Investigation for FrontEnd DC/DC Power Conversion for Distributed Power System”, by Bo Yang ina dissertation submitted to the Faculty of the Virginia PolytechnicInstitute and State University, Sep. 12, 2003. Among such resonant modeconverters, an LLC converter is preferred for reasons explained in thedissertation.

An LLC power converter is also known for example from Blom et al. U.S.Pat. No. 6,437,994, issued Aug. 20, 2002 and entitled “LLC ConverterIncludes A Current Variation Detector For Correcting A FrequencyAdjusting Control Signal Of An Included Difference Detector”.

An LLC converter has two resonant frequencies, namely a series resonantfrequency and a parallel resonant frequency, and is typically designedto operate in a range between these resonant frequencies in which thegain of the circuit is negative, meaning that an increase in frequencydecreases the energy transferred to the output of the converter. Forexample with a half bridge topology, the half bridge current lags thehalf bridge voltage due to a primarily inductive nature of the resonanttank in this range, so that the LLC can be operated to advantage withZVS.

An LLC converter is thus operated with a variable frequency switchingwaveform, which is a substantially square waveform with dead times toavoid simultaneous conduction of the half bridge switches. A higherfrequency corresponds to a lighter load. Although a particular LLCconverter may be designed for operation over a relatively narrow rangeof frequencies, different LLC converters for use in differentapplications, and with potentially different input voltages, may berequired to operate in very different frequency ranges over a widefrequency band.

STMicroelectronics Application Notes AN2321, “Reference design: highperformance, L6599-based HB-LLC adapter with PFC for laptop computers”,August 2006 and AN2393, “Reference design: wide range 200 W L6599-basedHB LLC resonant converter for LCD TV & flat panels”, September 2006disclose cascaded PFC and half bridge LLC power converters each using anL6563 controller for the PFC converter and a separate L6599 resonantcontroller for the LLC converter. Reference is also directed in theserespects to STMicroelectronics data sheets L6563, “Advancedtransition-mode PFC controller”, November 2006 and L6599, “High-voltageresonant controller”, July 2006.

It is also known, from Balakrishnan et al. U.S. Pat. No. 6,249,876,issued Jun. 19, 2001 and entitled “Frequency Jittering Control ForVarying The Switching Frequency Of A Power Supply”, to reduce EMI(electromagnetic interference) emission by jittering the switchingfrequency of a switched mode power supply.

It is desirable to minimize the number of connections required for acontrol unit for an LLC converter, especially if the control unit isprovided as an integrated circuit (IC) whether or not the IC alsoprovides for control of a PFC converter. At the same time, it isdesirable to provide for full control of the LLC converter, includingfor example determination of minimum and maximum switching frequencies,closed loop frequency control within the range of these frequencies,converter current sensing for overload protection, and input voltagemonitoring for soft start of the LLC power converter.

In addition, it is necessary to maintain an accurate matching of theon-times of the switches of an LLC converter, over all of itspotentially very large range of possible switching frequencies. Whilethese on-times ideally would be exactly 50% of the period at anyswitching frequency, in practice, as is well known, it is necessary toprovide dead times which reduce the on-times to slightly below 50% toavoid simultaneous conduction of the switches at the switching times.Accordingly, it is desirable for the dead times also to be closelymatched. Furthermore, it is desirable that the dead times be minimizedfor any given switching frequency; this presents a problem in view ofthe wide range of possible switching frequencies of the LLC converter.

SUMMARY OF THE INVENTION

One aspect of the invention provides a method of controlling a switchingfrequency of a power converter having an output voltage that isdependent upon the switching frequency, comprising the steps of:producing a first current that is dependent upon the output voltage;producing a second current corresponding to a desired maximum value ofthe first current; limiting the first current to the desired maximumvalue in dependence upon the second current; and producing a controlsignal for the power converter with a frequency determined by the firstcurrent.

Preferably the step of producing a control signal for the powerconverter comprises mirroring the first current.

The step of producing a control signal for the power converterpreferably comprises cyclically charging a capacitor with a currentdependent upon the first current and discharging the capacitor inresponse to it's voltage being charged to a threshold voltage thereby toproduce a sawtooth voltage waveform. The method can further comprise thestep of varying the charging current of the capacitor in a pseudo-randommanner, to facilitate a reduction in electromagnetic interference.

Preferably the method further comprises the step of producing twocomplementary switch control signals, constituting said control signalfor the power converter, for controlling two switches of the powerconverter for conduction in alternate cycles of the sawtooth waveformwith dead times between the conduction times of the two switches.

It is desirable to minimize such dead times, which are provided betweenconduction times of complementary switches of a power converter forwhich simultaneous conduction must be avoided, for example in a halfbridge power converter topology. This is especially the case where, forcascaded PFC and LLC converters in which each dead time determines akeep-out zone for switching of the PFC converter, this dead time limitsthe duty cycle range of the PFC converter. An optimum dead time isdependent upon a normal frequency range of the resonant mode converter,which may vary within a wide frequency band.

An embodiment of the invention facilitates this by including the step ofdetermining each dead time in dependence upon the second current.

The method preferably includes the step of providing a desired minimumvalue of the first current.

In an embodiment of the invention, the step of limiting the firstcurrent comprises the steps of: coupling differential inputs of anamplifier respectively to a voltage reference and a junction point towhich the first current is supplied; mirroring the second current via afirst transistor to a second transistor; conducting the first currentvia a third transistor to the second transistor; controlling the thirdtransistor in dependence upon an output of the amplifier; and changing avoltage at the junction point in response to a change of voltage at theoutput of the amplifier.

The method preferably includes the step of modifying the first currentby a current of a capacitor being charged via a resistor to change theswitching frequency of the power converter for soft starting of thepower converter.

Another aspect of the invention provides a control unit for a resonantmode converter having an output voltage that is dependent upon aswitching frequency of the converter, comprising: a feedback circuit forproviding a first current dependent upon the output voltage of theconverter; a resistor for producing a second current from a referencevoltage; a circuit for limiting the first current to the second current;and an oscillator circuit for producing a control signal for theconverter at a frequency dependent upon the first current thereby tocontrol said output voltage.

Preferably the circuit for limiting the first current to the secondcurrent comprises a current minor circuit for minoring the secondcurrent.

The oscillator circuit can comprise a capacitor, a current mirrorcircuit responsive to the first current for supplying a charging currentto the capacitor, and a comparator circuit responsive to the capacitorbeing charged to a threshold voltage for discharging the capacitorthereby to produce a sawtooth voltage waveform.

Preferably the control unit further comprises a circuit for producingtwo complementary switch control signals, constituting said controlsignal for the converter, for controlling two switches of the converterfor conduction in alternate cycles of the sawtooth waveform, and a timerfor producing dead times between the two complementary switch controlsignals. Advantageously the timer is responsive to the second currentfor determining each dead time in dependence upon the second current.

The control unit can and include a resistor for providing a currentconstituting a minimum value of the first current.

The circuit for limiting the first current to the second current cancomprise: an amplifier having differential inputs coupled respectivelyto a voltage reference and a junction point to which the first currentis supplied; a current mirror comprising a first transistor to which thesecond current is supplied and a second transistor; a third transistorvia which the first current is conducted to the second transistor, thethird transistor being controlled by an output of the amplifier; and acircuit for changing a voltage at the junction point in response to achange of voltage at the output of the amplifier.

The control unit preferably includes a capacitor in series with aresistor for modifying the first current by a charging current of thecapacitor for soft starting of the converter.

the invention also extends to the combination of a resonant modeconverter, having an output voltage that is dependent upon a switchingfrequency of the converter, and a control unit as recited above arrangedto control a switching frequency of the converter with said controlsignal. Preferably the resonant mode converter comprises an LLCconverter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and aspects thereof will be further understood from thefollowing description by way of example with reference to theaccompanying drawings, in which:

FIG. 1 schematically illustrates a power supply arrangement, includingcascaded PFC and LLC power converters and a control arrangement for theconverters, in accordance with an embodiment of the invention;

FIG. 2 illustrates in a block diagram parts of one form of a PFC and LLCcontrol unit of the control arrangement of FIG. 1;

FIGS. 3, 4, and 5 schematically illustrate parts of an LLC control unitof the PFC and LLC control unit of FIG. 2 in accordance with anembodiment of the invention; and

FIG. 6 schematically illustrates one form of a delay timer of thecontrol unit of FIG. 2.

DETAILED DESCRIPTION

A power supply arrangement as illustrated in FIG. 1 includes a PFC powerconverter 10 and an LLC power converter 11, the converters being shownwithin broken line boxes. The converters 10 and 11 are cascaded, apositive output voltage Vp of the PFC converter 10, produced on a line12 relative to a zero-volt (0V) line 13 connected to ground as shown,being connected as an input voltage for the LLC converter 11. Thecascaded PFC and LLC power converters 10 and 11 are controlled asdescribed further below by a PFC and LLC control unit 14, which has aground connection Gnd connected to the line 13.

AC power supplied to an input of the power supply arrangement isrectified by a diode bridge 15. A positive rectified AC output of thediode bridge 15 is coupled via a line 16 to a positive voltage input ofthe PFC converter 10, and a return path is provided from the 0V line 13to the diode bridge 15 via a current sensing resistor 17. By way ofexample, the line 16 may have a peak voltage in a range of about 125V toabout 360V, depending on a voltage of the AC power, and the voltage Vpon the line 12 may be about 385V.

The PFC converter 10 shown in FIG. 1 comprises a conventional boostconverter including an input inductor 18 and a diode 19 coupled inseries between the line 16 and the line 12, a controlled switch 20,typically constituted by a MOSFET, coupled between a junction of theinductor 18 with the diode 19 and the 0V line 13, and an outputcapacitor 21 coupled between the lines 12 and 13. The switch 20 iscontrolled to be opened and closed by an output P of the control unit14. Another output S of the control unit 14, not connected in FIG. 1, isprovided for complementary control (with dead times) of a secondaryswitch (not shown) which may be provided in other forms of PFCconverter.

A voltage divider comprising resistors 22 and 23 connected in seriesbetween the lines 12 and 13 supplies to a voltage feedback input Vfb ofthe control unit 14 a voltage proportional to the output voltage Vp ofthe PFC converter 10. Within the control unit 14, this voltage issupplied to a transconductance amplifier having an output coupled to acompensation point Vcom of the control unit 14, from which a capacitor24, and a resistor 25 in series with a capacitor 26, are connected toground or 0V. A negative voltage (relative to ground or 0V), produced atthe junction of the current sensing resistor 17 with the diode bridge 15and proportional to input current of the PFC converter 10, is coupled toanother input Vis of the control unit 14 via a low pass filterconstituted by a series resistor 27 and a shunt capacitor 28.

It is noted that the control unit 14 does not monitor the input voltageof the PFC converter 10, but only the input current and the outputvoltage Vp. The control unit 14 controls an off-time duty cycle Doff ofthe PFC converter switch 20 in accordance with:Doff=Vi/Vp=Re*Is/Vpwhere Vi is the input voltage on the line 16, Is is the input currentsensed by the current sensing resistor 17, and Re is the equivalent loadof the PFC converter reflected to its input, over a wide frequency rangeto provide a near-unity power factor for the power supply arrangement.

The LLC converter 11 has a half bridge topology comprising a primaryswitch 29 between the converter input voltage line 12 and a junctionpoint 30, and a secondary switch 31 between the junction point 30 and aline 32 of the converter. The switches 29 and 31, which typicallycomprise MOSFETs, are controlled in a complementary manner, with deadtimes so that they are not simultaneously conductive, by outputs A and Brespectively of the control unit 14. The line 32 is coupled to the 0Vline 13 via a current sensing resistor 33 providing a return path of theLLC converter 11, and is connected to an input OvL of the control unit14 to which it supplies a voltage proportional to input current of theLLC converter 11.

The junction point 30 is coupled to an output junction 36 of the LLCconverter 11 via a capacitor 34 and a series inductor 35, the junction36 being coupled via another inductor 37 to the line 32. The inductors35 and 37, and the capacitor 34, constitute the LLC components of theconverter 11. Outputs of the LLC converter 11 are taken from secondarywindings of a transformer 38, which has a primary winding connectedbetween the junction 36 and the line 32. In FIG. 1 the transformer 38 isrepresented as an “ideal” transformer, separate from the inductors 35and 37. In practice, part or all of the inductances of the inductors 35and 37 can be constituted by leakage and magnetizing inductances of thetransformer 38, so that functions of these inductors and the transformerare combined.

The transformer 38 can have any desired number of secondary windings;three secondary windings 39, 40, and 41 are shown by way of example inFIG. 1. The winding 39 has a centre tap, connected to a secondary sideground, and ends connected via full wave rectifier diodes 42 to anoutput 43. A smoothing capacitor 44 is connected between the output 43and the secondary side ground, so that the output 43 provides a DCvoltage output for equipment (not shown) powered by the power supplyarrangement. A voltage divider, comprising resistors 45 and 46 connectedin series between the output 43 and the secondary side ground, providesa voltage feedback for the LLC converter 11 as is further describedbelow.

The secondary winding 40 is coupled to a diode bridge 47 whose negativeoutput is connected to the primary side ground or 0V and whose positiveoutput, smoothed by a capacitor 48 connected between this positiveoutput and the 0V line 13, provides a supply voltage to an input Vcc ofthe control unit 14 for powering the control unit in a bootstrappedmanner. To this end, a high impedance resistor 49 is also connectedbetween the output line 12 of the PFC converter 10 and the input Vcc.

On connection of AC power to the power supply arrangement of FIG. 1, asmall current flows via the inductor 18, diode 19, and resistor 49 tocharge the capacitor 48, and the supply voltage at the input Vcc of thecontrol unit 14 rises. On this reaching a start-up voltage of, forexample, about 13V, this is detected by the control unit 14 whichaccordingly starts to drive the LLC converter 11, thereby to produce anoutput voltage via the secondary winding 40 and the diode bridge 47 tomaintain charge of the capacitor 48 to a desired operating voltage ofthe control unit 14, for example about 12V. The initial operation of thecontrol unit 14 reduces the charge of the capacitor 48, but notsufficiently to fall below a shut-down threshold voltage, of for exampleabout 8.5V.

The secondary winding 41, to which no connections are shown in FIG. 1,is representative of any number of other secondary windings of thetransformer 38 which may be used to provide other desired AC and/or DCoutputs at high or low voltages, as may be desired. It can beappreciated that functions of the secondary windings can be combined, sothat the transformer 38 can have one or more secondary windings.

The supply voltage at the input Vcc of the control unit 14 can be usedby the control unit 14 to provide a sufficiently high voltage to drivethe switches 20, 29, and 31 of the converters 10 and 11. In addition,the control unit 14 uses this supply voltage to produce at an outputVref a regulated supply voltage; this supply voltage is also used withinthe control unit 14 for powering most of its circuits. In addition,using the unregulated and/or regulated supply voltages the control unit14 powers a bandgap voltage reference (not shown) and derives variousthreshold voltages for use in operation of the control unit. By way ofexample, the regulated supply voltage is assumed to be 3.3V as shown inFIG. 1, and other voltages and voltage ranges referred to below aregiven in the context of this supply voltage.

A resistor 50 is connected between the output Vref of the control unit14 and an input Fmax of the control unit, to which it supplies a currentwhich determines a desired maximum switching frequency of the LLCconverter 11. Another resistor 51 is connected between the output Vrefof the control unit 14 and an input Fdbk of the control unit, to whichit supplies a current which determines a desired minimum switchingfrequency of the LLC converter 11. An electrically isolatingvoltage-to-current (V-I) converter 52 produces at its output an errorcurrent which is supplied via a series resistor 53 and a diode 54 to theinput Fdbk of the control unit 14 for feedback control of the frequencyof the LLC converter 11 within the range determined by the resistors 50and 51. This feedback error current is proportional to a differencebetween the voltage at the junction between the resistors 45 and 46,supplied to the converter 52 and representing the voltage at the DCoutput 43, and a reference voltage (not shown), and can be produced in afrequency compensated manner for example along the lines shown in FIG. 1of Application Note AN2321 referred to above.

An additional circuit, comprising a resistor 55 in series with acapacitor 56 between the input Fdbk and the output Vref of the controlunit 14, and optionally with a diode 57 in parallel with the resistor 55as shown in FIG. 1, provides for a soft start of the LLC converter 11under no-load or light load conditions, whereby the switching frequencyis reduced gradually from its maximum to a normal operating value.

FIG. 2 shows a block diagram of parts of one form of the PFC and LLCcontrol unit 14 of the power supply control arrangement of FIG. 1. Theseparts comprise a PFC control unit 60, an LLC control unit 61, an edgecontrol unit 62, a delay timer 63, a PFC output stage 64, and an LLCoutput stage 65. For simplicity other parts of the control unit 14, suchas for voltage regulation, producing desired threshold voltages,programming desired settings, and test purposes, are not shown.

Except for the connections Gnd, Vcc, and Vref which are not shown inFIG. 2, FIG. 2 shows the same external connections of the control unit14, using the same references, as FIG. 1. These references are also usedto refer to signals at the respective connections. FIG. 2 also showsvarious signals that are produced within and exchanged among variousparts of the control unit in operation, as described further below.Functions of the blocks shown in FIG. 2 and the related signals arebriefly described as follows.

The PFC control unit 60 is supplied with the PFC current sensing voltageVis and the PFC feedback voltage Vfb, and also has a connection to thecompensation point Vcom to which the components 24 to 26 are connectedas described above. These components are selected for a voltage at thepoint Vcom of typically 0.5 to 2.5V with a PFC control loop bandwidth ofthe order of about 10 to 20 Hz. The PFC control unit 60 compares thefeedback values Vis and Vfb with over-current and over-voltage thresholdvalues respectively, and in response to an over-current or over-voltagecondition of the PFC converter 10 determined by these comparisons itproduces a PFC fault signal Pflt which is supplied to the edge controlunit 62. The PFC control unit 60 also compares the feedback voltage Vfbwith an inhibit threshold voltage, and in response to an under-voltagecondition (e.g. in the event of AC brown-out or failure) determined bythis comparison produces an inhibit signal Inhib which is supplied tothe LLC control unit 61, the edge control unit 62, and the PFC outputstage 64.

In normal operating conditions, the PFC control unit 60 processes thefeedback signals Vis and Vfb to produce a signal Pmul, which is suppliedto the edge control unit 62, which is directly proportional to theoff-time duty cycle Doff required for the PFC converter 10 at anyinstant to provide the desired power factor correction in accordancewith the above equation for Doff. Thus throughout each rectified ACcycle of the PFC input voltage on the line 16 in FIG. 1, the off-timeduty cycle Doff, as represented by the signal Pmul, is varied by the PFCcontrol unit 60 to present an equivalent substantially resistive load tothe AC supply. By way of example, the signal Pmul can have a value from0 to 2.0V for representing off-time duty cycles from 0 to 100%.

The PFC control unit 60 can optionally use a ramp signal Lrmp, which isproduced by the LLC control unit 61 as described below, which can besupplied to the PFC control unit 60 as shown by a dashed line in FIG. 2.

The LLC control unit 61 is supplied with the signal Fdbk, which asdescribed above is a current representing an error voltage of the LLCconverter, and uses this to produce a controlled frequency squarewaveform clock signal Lclk which is supplied to the LLC output stage 65,and also to the edge control unit 62. The LLC control unit 61 alsoproduces a sawtooth or ramp signal Lrmp which is supplied to the edgecontrol unit 62 and, optionally as described above, to the PFC controlunit 60. For example the ramp signal Lrmp has an amplitude from 0 to2.0V and a frequency which is twice the frequency of the clock signalLclk. As indicated above, a minimum frequency of the LLC clock signalLclk is set by a minimum current supplied to the input Fdbk via theresistor 51, and a maximum frequency of the LLC clock signal Lclk is setby the resistor 50 supplying a current via the input Fmax to a currentmirror arrangement in the LLC control unit 61. For example the maximumfrequency may be set to a value about 2 or 3 times a normal LLCoperating frequency for a particular application, with the minimumfrequency being lower than this normal operating frequency. The normaloperating frequency typically is in a narrow frequency range, but may beselected from a wide frequency band, for example of the order of about50 kHz to about 1 MHz, for any particular application of the LLCconverter.

The LLC control unit 61 also produces a signal DTi for the delay timer63, this signal being a current that is produced by the current minorarrangement in the LLC control unit 61 in dependence upon the currentsupplied to its input Fmax. The delay timer 63 determines a dead time independence upon the current signal DTi, so that the dead time isadjusted for the wide range of possible LLC frequencies.

In addition, the LLC control unit 61 is supplied with the inhibit signalInhib to inhibit generation of the signals Lrmp and Lclk when the signalInhib is asserted. The LLC control unit 61 is further supplied via theinput OvL with the voltage dropped across the resistor 33 andrepresenting input current of the LLC converter 11, and compares thiswith at least one threshold to determine a possible overload conditionof the LLC converter, in response to which it produces an LLC faultsignal Lflt which is supplied to the LLC output stage 65. The LLCcontrol unit 61 is also supplied with the PFC feedback voltage signalVfb, which it compares with a threshold to enable start-up of the LLCconverter only when the PFC converter output voltage Vp is above aselected level, for example 360V. A soft start function in the LLCcontrol unit 61 operates in conjunction with the components 55 to 57 inFIG. 1 as indicated above to provide a soft start when the LLC converteris enabled and after any overload fault.

The edge control unit 62 compares the duty cycle signal Pmul with theLLC ramp signal Lrmp to produce a PFC PWM signal Ppwm with the desiredduty cycle, this signal being supplied to the PFC output stage 64. Thesignal Ppwm is harmonically related to the LLC clock signal Lclk, whichis also supplied to the edge control unit 62, conveniently in a 1:1 orsame-frequency relationship. The edge control unit 62 produces thesignal Ppwm with edges or transitions that are timed to avoid coincidingwith edges of the signal Lclk, for minimum interference, and with aphase for maximum efficiency of the power supply arrangement. To thisend the edge control unit 62 is also supplied with a signal Ldtrproduced by the LLC output stage 65 as described below, and which ishigh during dead times of the LLC output stage. The edge control unit 62is further supplied with the signals Pflt and Inhib, in response toeither of which it inhibits the signal Ppwm.

The delay timer 63 is responsive to a PFC delay time request signal Pdtrsupplied to it from the PFC output stage 64, or an LLC delay timerequest signal Ldtr supplied to it from the LLC output stage 65, toproduce a delay time done signal DTd, which is supplied to each of theseoutput stages 64 and 65, after a delay time that is determined asindicated above by the signal DTi, whereby the delay time is adjusted tosuit the normal operating frequency of the LLC converter 11 (and theswitching frequency of the PFC converter 10 which is here assumed to bethe same).

The PFC output stage 64 comprise a level shifter and gate driver forproducing the output P for driving the primary switch 20 of the PFCconverter 10 in accordance with the signal Ppwm and unless it isinhibited by the signal Inhib, with a similar arrangement for drivingthe output S in a complementary manner, with dead times, to avoidundesired simultaneous conduction of PFC converter switches, provided bythe delay timer 63 as described above. The PFC output stage 64 caninclude more complex arrangements for producing various relative timingsof its output signals P and S to suit different switching arrangementsthat may be required for different types of PFC converter.

The LLC output stage 65 also comprises level shifters and gate driversfor producing its output signals A and B for driving the switches 29 and31 respectively of the LLC converter 11, unless these are inhibited bythe signal Lflt, at the frequency of the signal Lclk and with deadtimes, to avoid simultaneous conduction of the switches 29 and 31,provided by the delay timer 63 as described above.

Particular forms of the LLC control unit 61 and the delay timer 63 aredescribed in greater detail and by way of example below. Particularforms of other parts of the PFC and LLC control unit 14 are described ingreater detail and by way of example in the related applicationsreferred to above.

It is noted that the LLC control unit described below can in somerespects be compared with the STMicroelectronics L6599 controller asdescribed in the data sheet for that device referred to above. Asdescribed in particular in section 7 of that data sheet, one pin (pin 4)of the L6599 controller is held at a reference voltage while sourcing acurrent that determines the frequency of an oscillator, and hence theswitching frequency of a controlled resonant mode converter. The currentis determined by a feedback signal to a photo-transistor and is limitedto a maximum value, determining a maximum frequency of the oscillator,by a resistor RFmax in series with the photo-transistor, and has aminimum value, determining a minimum frequency of the oscillator, set byanother resistor RFmin from the pin to ground. A resistor-capacitorcircuit from the pin to ground facilitates providing a soft-startfunction, using another pin (pin 1) connection for discharging thecapacitor of this circuit. A further pin (pin 3) provides for connectionof a main capacitor of the oscillator.

In this known controller, the main capacitor of the oscillator isalternately charged and discharged so that its voltage varies inaccordance with a triangular waveform with approximately equal voltageramps up and down which determine the on-times of the converterswitches. However, as shown in FIG. 21 of the data sheet, charging anddischarging currents of this main capacitor flow via different paths andthrough transistors of opposite polarity types, so that they may not beprecisely matched and consequently the on-times of the converterswitches may undesirably be unequal.

In addition, as shown in the block diagram on the first page of thisdata sheet a dead time block is used to determine the switching deadtimes, specified in Table 4 of the data sheet as a minimum of 0.2,typically 0.3, and a maximum of 0.3 microseconds. Thus in thiscontroller the dead time is fixed regardless of the switching frequencyof the controlled converter as determined by the oscillator.

Referring again to the accompanying drawings, FIGS. 3, 4, and 5schematically illustrate parts of a particular form of the LLC controlunit 61. FIG. 3 shows parts of the control unit 61 for producing acontrol current signal Limi for controlling the frequency of the LLCconverter, a clamp signal Clmp which is described further below, and thecurrent signal DTi referred to above. FIG. 3 also shows the components50, 51, and 54 to 57 connected to the output Vref and the inputs Fdbkand Fmax in the same manner as in FIG. 1. FIG. 4 shows an oscillatorarrangement of the LLC control unit 61 for producing the signals Lrmpand Lclk in dependence upon the current signal Limi. FIG. 5 showsoverload protection and slow start parts of the LLC control unit 61.

Referring to FIG. 3, the LLC control unit 61 includes a current minorarrangement comprising N-channel transistors 70 to 73. The transistor 70is diode-connected with its gate and drain connected to the input Fmax,and hence via the resistor 50 to the 3.3V supply voltage Vref.Consequently, a fixed current Ifmax, determined by the resistance of theresistor 50 and a voltage drop across this resistor, is conducted toground or 0V via the transistor 70. This current Ifmax determines amaximum frequency of the LLC converter 10 as further described below,and can be determined by appropriate selection of the resistor 50 to beanywhere within the wide frequency band in which the LLC converter maybe desired to operate. Mirroring of this current Ifmax enables otherparameters to be determined suitably for the maximum frequency, andhence the frequency range, for operation of any particular LLCconverter. Such parameters include the delay time, determined from themirrored current DTi, as further described below. It is noted incontrast that in the known L6599 arrangement as discussed above, theresistor RFmax only limits feedback current and hence maximum frequency,and does not permit determination of any other parameters.

The drain voltage of the transistor 70 is typically 0.6 to 0.9V. A moreprecise voltage at the input Fmax, and hence a more precise setting ofthe current Ifmax, can alternatively be provided by an amplifierarrangement coupled to the input Fmax, for example similar to thearrangement of the amplifier 74 in relation to the input Fdbk asdescribed below.

The current Ifmax in the transistor 70 is mirrored by the transistor 73to produce the current DTi for the delay timer 63. As described furtherbelow, a ramp generator in the delay timer 63 has a similar form to aramp generator of the LLC control unit oscillator. Consequently the deadtime determined by the delay timer 63 is adjusted according to thecurrent Ifmax to be suitable for the applicable LLC clock frequencywithin the wide frequency band, and there is a coarse compensationbetween characteristics of the delay timer 63 and the LLC clockfrequency.

The input Fdbk is connected to a non-inverting input of a differentialamplifier 74, to an inverting input of which is supplied a voltage Vbg,for example about 1.25V from the bandgap reference voltage. An output ofthe amplifier 74 is connected to the gates of two N-channel transistors75 and 76, which have a 10:1 current ratio as indicated in FIG. 3, andto a non-inverting input of a comparator 77, to an inverting input ofwhich is supplied a voltage clamp comparison voltage Vcl and whichproduces the clamp signal Clmp at its output. The transistor 75 has itssource connected to the drain of the transistor 71 and its drainconnected to the input Fdbk, which is also coupled to the 3.3V supplyvoltage by a P-channel transistor 78 controlled by an active-low softstart signal SSn supplied to its gate. The transistor 76 is connected inthe drain path of the transistor 72 to another current mirror formed byP-channel transistors 79 and 80, to produce the control current signalLimi.

In a steady operating state in which the signal SSn is high and thecapacitor 56 has a constant charge, the components 55 to 57 and 78 haveno effect. The amplifier 74 and the transistor 75 form a closed loopwhich normally acts to maintain the voltage Vbg at the input Fdbk. Asdescribed above, a current proportional to an output voltage derivedfrom the LLC converter 11 is supplied via the diode 54 to the inputFdbk. The resistor 51 also supplies to the input Fdbk a current equal tothe voltage, normally Vref-Vbg, across this resistor divided by theresistance of this resistor 51. Thus a current Ifdbk equal to the sum ofthese input currents is normally supplied to the input Fdbk and isconducted to ground or 0V via the transistors 75 and 71. This currentIfdbk is mirrored in a 10:1 ratio by the transistor 76 (the transistor76 passes a current equal to Ifdbk/10), and the resulting current ismirrored as the control signal current Limi to determine the LLC clockfrequency as described below. Consequently, the control signal currentLimi, and hence the LLC clock signal frequency, is controlled by thefeedback current through the diode 54, and a minimum current and hence aminimum frequency is determined by the resistance of the resistor 51.Thus the minimum frequency can also be determined to be anywhere withinthe wide frequency band in which the LLC converter may be desired tooperate by appropriate selection of the resistor 51.

Thus the two resistors 50 and 51, externally of an integrated circuitimplementing the control unit, determine maximum and minimum frequenciesof the LLC converter 11 anywhere within a wide band of possiblefrequencies as described above, using only two integrated circuitinputs. One (Fdbk) of these is also used for the feedback signal, andthe other (Fmax) provides a current that can be used to determine notonly the maximum frequency, but also the dead time and other parametersas may be desired.

In the normal operating condition described above, the current Ifdbkpassed by the transistors 75 and 71 is less than the current Ifmaxpassed by the transistor 70, and the output voltage of the amplifier 74is less than the voltage Vcl so that the signal Clmp is low. An increaseof the feedback current via the diode 54, and hence of the currentIfdbk, corresponding to an increase in the output voltage of the LLCconverter at the output 43, for example due to a reduced LLC converterload, results in an increased control signal current Limi, and hence anincreased frequency of the LLC clock frequency, which produces a reducedoutput of the LLC converter in accordance with its negativegain-frequency characteristics.

Current through the transistors 75 and 71 is limited to the currentIfmax through the transistor 70. If the current Ifdbk attempts to risebeyond this, then the closed loop formed by the amplifier 74 and thetransistor 75 can no longer maintain the voltage Vbg at the input Fdbk,and the voltage at this input Fdbk rises. Consequently the outputvoltage of the amplifier 74 rises above the voltage Vcl, and thecomparator 77 produces a high level of the clamp signal Clmp, inresponse to which (via an OR gate 106 as shown in FIG. 5) the signalLflt is asserted to inhibit the LLC output stage 65 in FIG. 2, resultingin the output of the LLC converter 10 decreasing. Thus the LLC converterfrequency is limited to the maximum frequency set by the resistor 50.

The signal SSn is asserted (with a low level) on start-up of the LLCconverter and in response to fault conditions, as described furtherbelow, to turn on the transistor 78 for at least a minimum delaydetermined by a counter, thereby pulling the input Fdbk to the 3.3Vsupply voltage Vref and discharging the capacitor 56 via the resistor55, or quickly via the diode 57 if it is present. The high level of theinput Fdbk results in a minimal current into the input Fdbk, a highlevel of the signal Clmp being produced or maintained, and the signalcurrent Limi being at its maximum of Ifmax/10 corresponding to themaximum frequency of the LLC clock signal Lclk.

On removal of the low level of the signal SSn, the capacitor 56 ischarged via the resistor 55, the charging current flowing into the inputFdbk and forming part of the current Ifdbk. As the capacitor 56 charges,the current Ifdbk falls gradually from Ifmax to a lower stable value,the frequency of the LLC clock signal Lclk is accordingly reducedgradually from its maximum value to a lower stable operating value, andthe high level of the signal Clmp is ended, the voltage at the inputFdbk again becoming equal to Vbg through the feedback loop provided bythe amplifier 74 and the transistor 75. The resistor 55 and thecapacitor 56 can provide a relatively long time constant, for example ofthe order of about 100 s or more. This soft start function reduces theload that the LLC converter 11 presents, on start-up, to the PFCcapacitor 21.

As the current Ifdbk provided at the input Fdbk determines the switchingfrequency of the LLC converter 11 and hence its output voltage, the LLCcontrol unit 61 can be sensitive to noise at this input. Noisesensitivity can be reduced in a variety of ways such as debouncing orlow pass filtering the current at this input, or reducing bandwidth ofthe circuit including the amplifier 74. However, it is observed thatsome ripple at this input Fdbk, for example 120 Hz ripple from the ACsupply, may be beneficial in producing a spectral spread of the LLCconverter switching frequency which can potentially reduceelectromagnetic interference. Noise at the input Fdbk can also occurdifferently in a pattern of alternate cycles of the LLC oscillator,corresponding to the two different switching states of the LLCconverter, possibly adversely affecting the necessary equal timing ofthese states. This disadvantage can be avoided by providing a sample andhold function at the input Fdbk, so that the same value of the feedbackcurrent Ifdbk is used for determining at least two successive cycles ofthe LLC oscillator and hence facilitating equal timing of the two LLCconverter switching states.

Referring to FIG. 4, the oscillator arrangement of the control unit 61,for producing the signals Lrmp and Lclk in dependence upon the controlsignal current Limi, comprises a current minor 81 formed by N-channeltransistors, a current mirror 82 formed by P-channel transistors andhaving multiple outputs (for example with binary weightings) which areselectively connected in parallel by switches 83, a capacitor 84, anN-channel transistor 85, an OR gate 86, a pulse stretcher 87, acomparator 88, and a flip-flop 89 having a clock input which is shown inconventional manner, an inverting output −Q, a data input D connected tothis output −Q, and a reset input R.

As shown in FIG. 4, the current Limi is mirrored by the current mirror81, an output current of which is mirrored by the current mirror 82 toproduce a current for charging the capacitor 84. The switches 83 areprogrammed, by one-time programming (OTP) represented by a block 90, forcalibration of the current supplied by the current minor 82 thereby tocompensate for manufacturing process variations, a dominant one of whichis typically the capacitance of the capacitor 84. In addition, theswitches 83 can be controlled by a pseudo-random (P-R) signal source 91to dither the charging current of the capacitor 84, and hence to ditherthe LLC switching frequency produced as described below, in a manner toreduce EMI by spreading its spectrum.

A voltage to which the capacitor 84 is charged, constituting the LLCramp signal Lrmp, is supplied to a non-inverting input of the comparator88, an inverting input of which is supplied with a comparison voltage,of 2.0V as shown, which corresponds to the maximum amplitude of thesignal Lrmp. An output of the comparator 88 is supplied to the clockinput of the flip-flop 89, whose −Q output constitutes the LLC clocksignal Lclk, and drives the pulse stretcher 87 via the OR gate 86. Anoutput of the pulse stretcher 87 controls the gate of the transistor 85,which has its drain-source path connected in parallel with the capacitor84. The inhibit signal Inhib is supplied to the reset input of theflip-flop 89 and to a second input of the OR gate 86, to inhibitgeneration of the signals Lrmp and Lclk when the signal Inhib is high.

Consequently, the capacitor 84 is repeatedly charged linearly at a rateproportional to the signal current Limi and calibrated and optionallydithered by the switches 83, until it reaches the maximum voltage of2.0V and the comparator 88 produces a high output, toggling theflip-flop 89 and turning on the transistor 85 to discharge the capacitor84 rapidly to 0V, restoring a low level at the output of the comparator88. The pulse stretcher 87 provides a sufficiently long on period, offor example 50 ns or less, for the transistor 85 to discharge thecapacitor 84 fully, while still providing a sufficiently fast edge ofthe sawtooth or ramp signal Lrmp.

It can thus be appreciated that the LLC ramp signal Lrmp is a linearsawtooth at a frequency determined by the feedback current Ifdbk, andthat the LLC clock signal Lclk is a square waveform at half thisfrequency, up to a maximum clock frequency corresponding to the maximumcurrent Ifmax. Further, this control arrangement enables the clockfrequency to be varied over the wide band of possible frequencies of theLLC converter 11.

FIG. 5 shows overload protection and soft start parts of the controlunit 61. These parts comprise fast and slow overload (OVL) shut-offcircuits 100 and 101 respectively, OR gates 102, 105, and 106, acomparator 103, an inverter 104, an edge-triggered RS flip-flop 107, adelay counter 108, and a NOR gate 109.

The circuit 100 compares the voltage of the OvL input with a thresholdrepresenting a relatively high overload of the LLC converter 11, andproduces a high output via the OR gate 102 immediately if the thresholdis exceeded. The circuit 101 compares the voltage of the OvL input witha lower threshold representing a smaller overload of the LLC converter11, and produces a high output via the OR gate 102 if this threshold isrepeatedly exceeded. In either case a high output of the gate 102indicates an overload condition. The comparator 103 compares the voltageof the input Vfb, representing the output voltage of the PFC converter10 which is the input voltage of the LLC converter 11, with a shutdownthreshold Vsd below which the LLC converter 10 is to be turned off, inthis event producing a low output which is inverted by the inverter 104.The outputs of the gate 102 and the inverter 104 are combined in the ORgate 105, the output of which is supplied to the gates 106 and 109. TheOR gate 106 is also supplied with the clamp signal Clmp, its outputconstituting the signal Lflt. A rising edge of this output of the gate106 sets the flip-flop 107 to enable the delay counter 108. The delaycounter counts a desired number, for example 1024, of cycles of the LLCclock signal Lclk and then produces an output which resets the flip-flop107. The output Q of the flip-flop 107 is also connected to an input ofthe NOR gate 109, the output of which constitutes the active-low softstart signal SSn.

Consequently, in the event of an overload of the LLC converter 11, anunder-voltage at the output of the PFC converter 10, or a clamped stateas described above, the signal Lflt is asserted to inhibit the output ofthe LLC converter 11, and a low value of the signal SSn is produced topull the input Fdbk high as described above, the latter condition beingmaintained for at least the period counted by the delay counter 108 toallow time for the capacitor 56 to be fully discharged. At the end ofthis period, when the flip-flop 107 is reset, the low level of thesignal SSn is ended if the output of the gate 105 is low, i.e. if thereis no overload or under-voltage condition, but the high level of thesignal Clmp remains while the LLC clock signal gradually falls from itsmaximum frequency to a stable operating frequency as described above.The signal Clmp then goes low to end the high level of the signal Lfltand enable the LLC output stage 65.

FIG. 6 shows a particular form of the delay timer 63, in which thecurrent DTi is mirrored by a current minor 171, constituted by P-channeltransistors with multiple outputs selectively connected in parallel byprogrammable switches 172, to produce a calibrated current Di forcharging a capacitor 173. The switches 172 are programmed to compensatefor manufacturing process variations, in particular for the capacitor173.

An N-channel transistor 174 has its drain-source path in parallel withthe capacitor 173 and its gate connected to the output of a NOR gate 175whose inputs are supplied with the signals Pdtr and Ldtr, so that avoltage across the capacitor 173 is held at zero until one of thesignals Pdtr and Ldtr goes high at the start of a requested dead time.Then the capacitor 173 is charged, with its voltage, supplied to anon-inverting input of a comparator 176 to an inverting input of whichis supplied a threshold voltage of 2.0V as shown, rising linearly untilit reaches the threshold at the end of the dead time, the comparatorstate then changing to produce a high value at its output constitutingthe signal DTd. In response to the high value of the signal DTd, a highvalue of the signal Ldtr is ended in the LLC output stage 65 for exampleas described below; a high value of the signal Pdtr is similarly endedin the PFC output stage 64. It is observed that the signals Pdtr andLdtr can not both be high simultaneously.

It will be appreciated that the form of the delay timer 63 as shown inFIG. 6 is similar to the form of the oscillator as shown in FIG. 4, sothat in any individual integrated circuit implementing both of thesethere can be an approximate correlation of their characteristics. As aresult, the delay time can be well matched to the maximum switchingfrequency of the LLC converter.

Although the above description relates to an LLC converter using a halfbridge topology, the invention can also be applied to other resonantmode converters and to other power converter topologies, for example toa full bridge topology in a similar manner. It can also be applied in asimilar manner to controlling the switching of other power converters,not shown, which may be provided in addition to the PFC and LLCconverters, for example to one or more flyback or other PWM convertersthat may be desired for providing additional supply voltages such as maybe desired for standby and/or operating power for equipment powered bythe power supply arrangement.

Although particular forms of the power supply arrangement and controlunits are described above by way of example, numerous modifications,variations, and adaptations may be made thereto.

1. A control unit for use in a resonant mode power converter, comprising: a current limiting circuit coupled to receive a first current representative of a power converter output and a second current generated in response to a reference voltage, the current limiting circuit coupled to limit the first current in response to the second current; and an oscillator coupled to receive the first current, the oscillator coupled to generate a control signal having a control frequency in response to the first current, wherein the power converter output is controlled in response to the control frequency.
 2. The control unit of claim 1 wherein the current limiting circuit comprises a first current mirror coupled to receive the first current and the second current, wherein the first current mirror is coupled to limit the first current in response to the second current.
 3. The control unit of claim 1 wherein the oscillator comprises: a second current minor coupled to generate a charging current in response to the first current; a capacitor coupled to the second current mirror, the capacitor coupled to be charged in response to the charging current; and a comparator coupled to the capacitor and coupled to receive a threshold voltage, wherein the capacitor is coupled to be discharged in response to the threshold voltage received by the comparator to generate a sawtooth control signal having the control frequency in response to the first current.
 4. The control unit of claim 3 further comprising a pseudo-random signal source coupled to the second current mirror to dither the charging current such that the control frequency is dithered in response to the pseudo-random signal source to reduce electromagnetic interference (EMI).
 5. The control unit of claim 3 further comprising a switch control signal generator coupled to receive the control signal having the control frequency, the switch control signal generator coupled to generate first and second complementary switch control signals in response to the control signal having the control frequency.
 6. The control unit of claim 5 further comprising two switches of the power converter coupled to receive the first and second complementary switch control signals, respectively, to control conduction of the two switches in alternate sawtooth control signal cycles to control the power converter output.
 7. The control unit of claim 6 further comprising a delay timer coupled to generate dead times between the first and second complementary switch control signals in response to the second current.
 8. The control unit of claim 1 further comprising a resistor coupled to a capacitor and coupled to a reference voltage to modify the first current to reduce gradually the control frequency from a maximum to a normal operating value during a soft start of the power converter.
 9. The control unit of claim 1 wherein the power converter is a resonant mode power converter comprising an LLC converter. 